Organic solvents such as perchloroethylene and other low-pressure liquid solvents have long been popular for use in cleaning systems such as dry cleaning systems. Recently, however, there are growing concerns that these solvents may harm the environment and pose occupational safety hazards. These concerns have led to an extensive search for alternative solvents that are less hazardous and systems for applying such solvents.
Some of this research has focused on systems utilizing solvents that are gases at low pressure. These systems may operate either under subcritical conditions such that the solvent is present as a liquid or under supercritical conditions such that the solvent is present as a supercritical fluid. Some of these systems utilize liquid carbon dioxide (CO2) as a cleaning solvent.
PCT Publication WO 99/13148 to Shore et al. describes a cleaning system using liquid CO2. Shore describes evacuating a cleaning chamber to remove air from the chamber. Shore also discusses filling the chamber with carbon dioxide gas from either a distillation vessel or a liquid CO2 storage tank as part of a prefill mode. Shore further describes how draining liquid carbon dioxide from the cleaning chamber leaves carbon dioxide gas in the chamber and discusses an apparatus for reclaiming this gas using a compressor and a condenser to return reliquified CO2 to a liquid storage tank.
The system described by Shore is inefficient making it expensive to operate and expensive to construct. For example, filling the cleaning chamber with CO2 gas from a distillation vessel requires that a distillation vessel be supplied and operated. Alternatively, using vaporization of the liquid CO2 in the storage tank requires the storage tank to contain a heater sized to provide make-up heat equal to the heat of vaporization of the liquid CO2 that is converted to vapor.
Furthermore, a condenser must be supplied which is sized to handle the extreme vapor loads experienced at the beginning of the vapor reclamation operation. Additionally, cooling must be supplied to this condenser. Other methods for removing the CO2 gas from the cleaning chamber such as venting to atmosphere, which results in loss of CO2 from the system, or sparging as described in PCT Publication WO 97/33031 to Taricco are similarly inefficient.
A small amount of air in the system may be beneficial, providing a partial pressure in the liquid CO2 storage tank and resulting in increased net positive suction head for the pump. However, the efficiency of the condenser can be drastically affected by even small amounts of air. Thus, a vacuum pump must be operated before each cycle to ensure that all air has been evacuated from the cleaning chamber.
Further inefficiencies occur in carbon dioxide cleaning systems that employ cleaning solutions comprising liquid carbon dioxide and other additives or detergents. To create a source of liquid CO2, these systems rely on evaporators or stills to separate additives and contaminants from the cleaning solution and generate CO2 vapor. Such stills and evaporators require heating elements, which must be sized to supply sufficient CO2 vapor and operated using steam or electricity.